This application claims the benefit of Japanese Patent applications No. 2001-127816 and No. 2001-127830 which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a zoom lens system having a high zoom ratio, a light weight, compactness and good cost performance and, in particular, to a compact standard zoom lens system having a high zoom ratio.
2. Related Background Art
In recent years, a so-called standard zoom lens with a zoom ratio of 3 to 4 including a wide-angle end state has steadily become smaller its dimension and lower the manufacturing cost. Since the lens system is mounted on the camera body as the regularly used lens, it is essential to have compactness, lightweight, satisfactory imaging performance, and reasonable manufacturing cost. In order to satisfy the above-described requirements, it is necessary for each lens group to have strong refractive power and to be thinner as much as possible.
For example, zoom lens systems having four lens groups of positive-negative-positive-positive arrangement from the object side have been proposed in Japanese Patent Application Laid-Open Nos. 1-229217, 8-248319, 9-101459, 2000-075204, 2000-187161, 7-113957, 2000-338401.
However, the zoom lens systems disclosed in the above-mentioned publications mainly have the zoom ratio of about 3 to 4. Even the zoom lens system having a high zoom ratio has been large in dimension, has a large number of elements. In addition, the optical performance has not been satisfactory. Accordingly, a compact zoom lens system, as the present invention, being downsized ultimately having a zoom ratio of more than 6.62, which corresponds to 28 mm to 200 mm in the 35 mm film format, has not been existed.
The present invention is made in view of the aforementioned problems and has an object to provide a zoom lens system with an angle of view 2xcfx89 of about 74.1xc2x0 to 11.8xc2x0 and a zoom ratio of about 6.6, being ultimately small in diameter and compact, being composed of less number of elements, and having high cost performance and superb optical performance.
According to one aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, at least one lens group, and a lens group Gm having positive refractive power as a whole. The focal length of the zoom lens system is varied by changing an air space between the first lens group G1 and the second lens group G2. The lens group Gm consists of, in order from the object, a positive lens component L1 having a convex surface facing to an image side and a negative lens component L2 having a concave surface facing to the object side. The following conditional expression (1) is satisfied:
0.10 less than xcexa3dw/ft less than 0.54xe2x80x83xe2x80x83(1)
where xcexa3dw denotes a distance along the optical axis between the vertex of the object side surface of the most object side lens element and that of the image side surface of the most image side lens element in a wide-angle end state and ft denotes the focal length of the zoom lens system in a telephoto end state.
In one preferred embodiment, the following conditional expression (2) is satisfied:
0.20 less than f1/ft less than 0.55xe2x80x83xe2x80x83(2)
where f1 denotes the focal length of the first lens group G1.
In one preferred embodiment, the following conditional expression (3) is satisfied:
0.03 less than |f2|/ft less than 0.20xe2x80x83xe2x80x83(3)
where f2 denotes the focal length of the second lens group G2.
In one preferred embodiment, the following conditional expression (4) is satisfied:
0.23 less than dpn/dm less than 0.90xe2x80x83xe2x80x83(4)
where dpn denotes a distance along the optical axis between the image side vertex of the positive lens component L1 and the object side vertex of the negative lens component L2 in the lens group Gm, and dm denotes a distance along the optical axis between the object side vertex of the most object side lens element and the image side vertex of the most image side lens element of the lens group Gm.
In one preferred embodiment, at least one aspherical surface is included in either the positive lens component L1 or the negative lens component L2 in the lens group Gm.
In one preferred embodiment, the at least one lens group consists only of a third lens group G3 having positive refractive power. The following conditional expression (5) is satisfied:
0.2 less than f3/fm less than 1.0xe2x80x83xe2x80x83(5)
where f3 denotes the focal length of the third lens group G3 and fm denotes the focal length of the lens group Gm.
In one preferred embodiment, at least one aspherical surface is included in both positive lens component L1 and negative lens component L2 in the lens group Gm. At least one of the aspherical surfaces has a shape that that positive refractive power of a single lens element becomes weak or negative refractive power of a single lens element becomes strong as the height goes from the optical axis to the periphery of the lens element.
In one preferred embodiment, the positive lens component L1 in the lens group Gm is constructed by double aspherical surfaces, and wherein the image side surface of the double aspherical lens has a shape that the curvature in the periphery of the effective aperture becomes larger than that on the optical axis.
In one preferred embodiment, the negative lens component L2 in the lens group Gm has at least one aspherical surface. The aspherical surface has a shape that the negative refractive power in the most peripheral part of the effective aperture becomes stronger than that on the optical axis.
In one preferred embodiment, the second lens group G2 has at least one negative lens element and at least one positive lens element. The following conditional expressions (6) and (7) are satisfied:
np less than 1.85xe2x80x83xe2x80x83(6)
vp less than 27xe2x80x83xe2x80x83(7)
where np denotes refractive index of the positive lens element at d-line and vp denotes Abbe number of the positive lens element.
According to another aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, at least one lens group, and a lens group Gm having positive refractive power. The focal length of the zoom lens system is varied by changing an air space between the first lens group G1 and the second lens group G2. The lens group Gm consists of, in order from the object, a positive lens component L1 having a convex surface facing to an image side and a negative lens component L2. The negative lens component L2 includes a positive lens element La and a negative lens element Lb. The following conditional expression (8) is satisfied:
0.10 less than xcexa3dw/ft less than 0.54 xe2x80x83xe2x80x83(8)
where xcexa3dw denotes a distance along the optical axis between the vertex of the object side surface of the most object side lens element and that of the image side surface of the most image side lens element in a wide-angle end state and ft denotes the focal length of the zoom lens system in a telephoto end state.
In one preferred embodiment, the following conditional expression (9) is satisfied:
0.20 less than f1/ft less than 0.85xe2x80x83xe2x80x83(9)
where f1 denotes the focal length of the first lens group G1.
In one preferred embodiment, the positive lens component L1 in the lens group Gm having a convex surface facing to the object has at least one aspherical surface. The aspherical surface has a shape that the curvature becomes small as the height goes from the optical axis to the periphery of the lens element.
In one preferred embodiment, the positive lens component L1 in the lens group Gm having a convex surface facing to the object has aspherical surfaces on both object side and image side. The object side surface of the aspherical surfaces has a shape that the curvature becomes large as the height goes from the optical axis to the periphery of the lens element. The image side surface of the aspherical surfaces has a shape that the curvature becomes small as the height goes from the optical axis to the periphery of the lens element.
In one preferred embodiment, the following conditional expression is satisfied:
0.10 less than nb-naxe2x80x83xe2x80x83(10)
where na denotes refractive index of the positive lens element La in the negative lens component L2 in the lens group Gm at d-line and nb denotes that of the negative lens element Lb in the negative lens component L2 in the lens group Gm at d-line.
In one preferred embodiment, the positive lens element La and the negative lens element Lb in the negative lens component L2 in the lens group Gm are cemented with each other.